Delay line ring oscillation apparatus

ABSTRACT

The delay line degradation protection architecture as build-in ring oscillation apparatus includes a two gates logical circuit, a buffer, a clock input buffer and a delay lock loop circuit. The two gates logical circuit receives a clock enable signal, specific mode signal, and delayed clock output signal. The two gates logical circuit performs a logical operating on the clock enable signal, the specific mode signal and the delayed clock output signal for generating a mode selecting signal. The buffer generates a feedback signal according to the mode selecting signal and a control signal. The clock input buffer decides whether to transport the input clock signal to an output end of the clock input buffer or not according to the feedback signal. The delay lock loop circuit generates the delayed clock output signal. A frequency of the feedback signal is adjusted according to the control signal.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention generally relates to a delay line degradationprotection architecture, especially with built-in ring oscillationapparatus, and more particularly to an in-system delay line ringoscillation apparatus.

2. Description of Prior Art

When a clock path including a delay lock loop circuits that were in usego into an asymmetric stress condition for the time the clock is nottoggled, especially conditions including slow exit power down,self-refresh, or any operations requiring delay lock loop circuit resetaftermath. It is hoped on not going into this type of non-clocking statefor long periods of time in the past, or hope even/odd number of randomevent durations are balanced.

In a dynamic random access memory (DRAM) application, for the issuementioned above, both duty cycle degradation and a tDQSCK (data signalskew to clock signal) timing shift due to degradation stress mismatchare exposed. The same issue can be seen in the clock distribution tree.The issues impact on DRAM lifetime severely considering servers fieldapplication randomness.

SUMMARY OF THE INVENTION

The present disclosure provides a delay line ring oscillation apparatusfor extending reliability lifetime concept.

The delay line ring oscillation apparatus provided by the presentdisclosure includes a two gates logical circuit, a buffer, a clock inputbuffer and a delay lock loop circuit. The two gates logical circuit hasthree input ends and an output end. Two of the three input endsrespectively receive a clock enable signal and a specific mode signal,and the third input end receiving a delayed clock output signal or afine delayed output signal. The two gates logical circuit performs alogical operating on the clock enable signal, the specific mode signaland the one of the delayed clock output signal and the fine delayedoutput signal for generating a mode selecting signal on the output endof the two gates logical circuit. The buffer receives the mode selectingsignal and generates a feedback signal according to the mode selectingsignal and a control signal. The clock input buffer receives thefeedback signal and an input clock signal. The clock input bufferdecides whether to transport the input clock signal to an output end ofthe clock input buffer or not according to the feedback signal. Thedelay lock loop circuit receives and delays the signal on the output endof the clock input buffer for generating the delayed clock outputsignal. Wherein, a frequency of the feedback signal is adjustedaccording to the control signal.

In an embodiment of the disclosure, the two gates logical circuit is anand-or-inverter (AOI) gate. The AOI gate has a NAND input end, a firstOR input end and a second OR input end, and an AOI output end. The NANDinput end receives a clock enable signal, the first OR input endreceives a specific mode signal, and the second OR input end receivesthe delayed clock output signal or the fine delayed output signal.

In an embodiment of the disclosure, the clock input buffer is a NANDgate. The NAND gate has a first input end a second input end and anoutput end. The first input end of the NAND gate receives the feedbacksignal, the second input end of the NAND gate receives the input clocksignal, and the output end of the NAND is coupled to the delay lock loopcircuit.

In an embodiment of the disclosure, the delay lock loop circuit is acoarse delay lock loop circuit.

In an embodiment of the disclosure, the delay line ring oscillationapparatus further includes a fine delay lock loop circuit. The input iscoupled to the coarse delay lock loop circuit. The fine delay lock loopcircuit receives a coarse delayed output signal from the coarse delaylock loop circuit. The fine delay lock loop circuit generates the finedelayed output signal according to the coarse delayed output signal.

In an embodiment of the disclosure, the delay line ring oscillationapparatus further includes a NOR gate. The NOR gate is coupled to thetwo gates logical circuit. Wherein, the NOR gate receives a power downsignal, a self-refresh signal and a test mode for negative biastemperature instability (NBTI) signal. The NOR gate is used to generatethe specific mode signal.

In an embodiment of the disclosure, the buffer includes a plurality ofinverting gates. The inverting gates are coupled in series. The firstinverting gate receives the mode selecting signal, and the lastinverting gate generates the feedback signal. Each of a plurality ofgate delays of each of the inverting gates is controlled by the controlsignal.

In an embodiment of the disclosure, each of the inverting gates includesa first transistor, a second transistor, a third transistor, a fourthtransistor, a first switch and a second switch. The first transistor hasa first end, a second end and a control end. The first end of the firsttransistor is coupled to a first reference voltage, and the control endof the first end is an input end of the inverting gate. The secondtransistor has a first end, a second end and a control end. The controlend of the second transistor is coupled to the control end of the firsttransistor, and the second end of the second transistor is coupled to asecond reference voltage. The third transistor has a first end, a secondend and a control end. The first end of the third transistor is coupledto the second end of the first transistor, and the second end of thethird transistor is an output end of the inverting gate. The firstswitch is coupled between the input end of the inverting gate and thecontrol end of the third transistor. The first switch is used to connectthe control end of the third transistor to the input end of theinverting gate or the second reference voltage according to the controlsignal. The fourth transistor has a first end, a second end and acontrol end. The first end of the fourth transistor is coupled to theoutput end of the inverting gate, and the second end of the fourthtransistor is coupled to the first end of the second transistor. Thesecond switch is coupled between the input end of the inverting gate andthe control end of the fourth transistor. The second switch is used toconnect the control end of the fourth transistor to the input end of theinverting gate or the first reference voltage according to the controlsignal.

In an embodiment of the disclosure, each of the inverting gates furtherincludes a first capacitor and a second capacitor. The first capacitoris coupled between the output end of each of the inverting gates and thefirst reference voltage. The second capacitor is coupled between theoutput end of each of the inverting gates and the first referencevoltage.

In an embodiment of the disclosure, the first capacitor is formed by aN-type transistor, and the second capacitor is formed by a P-typetransistor.

In an embodiment of the disclosure, the first transistor and the thirdtransistor are P-type transistors, and the second transistor and thefourth transistor are N-type transistors.

In an embodiment of the disclosure, when the first switch connects thecontrol end of the third transistor to the input end of the invertinggate, the second switch connects the control end of the fourthtransistor to the first reference voltage. When the first switchconnects the control end of the third transistor to the second referencevoltage, the second switch connects the control end of the fourthtransistor to the input end of the inverting gate.

In an embodiment of the disclosure, the amount of the inverting gates ofthe buffer is even.

Accordingly, the present disclosure provides the delay line ringoscillation apparatus with the two gates logical circuit and a bufferwhich may be used to adjust the frequency of the feedback signal.According to the specific mode signal, the two gates logical circuit,the buffer, the clock input buffer and the delay lock loop circuit forma loop with odd number gates, and a delayed clock output signal with aslow frequency and a stable duty cycle can be generated accordingly.That is, the asymmetrical degradation stress problem of the delay linering oscillation apparatus can be solved, and the system performance canbe extended by making the delay line ring oscillation apparatus safefrom negative bias temperature instability (NBTI) and/or positive biastemperature instability (PBTI) hazard.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a block diagram of a delay line ring oscillation apparatus 100according to an embodiment of the present disclosure.

FIG. 2 is a block diagram of a delay line ring oscillation apparatus 200according to another embodiment of the present disclosure.

FIG. 3 is a circuit diagram of the buffer 220 according to theembodiment of the present disclosure.

FIG. 4A and FIG. 4B respectively are circuit diagrams of the capacitorsC1 and C2 according to the embodiment of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present preferred embodimentof the invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

Please referring to FIG. 1, FIG. 1 is a block diagram of a delay linering oscillation apparatus 100 according to an embodiment of the presentdisclosure. The delay line ring oscillation apparatus 100 may be used inany synchronous electronic devices, including dynamic random accessmemory (DRAM). The delay line ring oscillation apparatus 100 includes atwo gates logical circuit 110, a buffer 120, a clock input buffer 130and a delay lock loop circuit 140. The two gates logical circuit 110 hasthree input ends and an output end, the three input ends of the twogates logical circuit 110 respectively receive a clock enable signalCLKENB, a specific mode signal SPCMD, and a delayed clock output signalDCK. The two gates logical circuit 110 performs a logical operating onthe clock enable signal CLKENB, the specific mode signal SPCMD and thedelayed clock output signal DCK for generating a mode selecting signalMSS on the output end of the two gates logical circuit 110.

The buffer 120 is coupled to the two gates logical circuit 110 and theclock input buffer 130. The buffer 120 receives the mode selectingsignal MSS from the two gates logical circuit 110, the buffer 120 alsoreceives a control signal CTRL. The buffer 120 generates a feedbacksignal FB according to the mode selecting signal MSS and the controlsignal CTRL, wherein, a frequency of the feedback signal FB may beadjusted according to the control signal CTRL.

The clock input buffer 130 is coupled between the buffer 120 and thedelay lock loop circuit 10. The clock input buffer 130 receives thefeedback signal FB and a input clock signal CKIN. The clock input buffer130 decides whether to transport the input clock signal CKIN to anoutput end of the clock input buffer 130 or not according to thefeedback signal FB.

The delay lock loop circuit 140 is coupled to the clock input buffer 130and the two gates logical circuit 110. The delay lock loop circuit 140receives and delays the signal on the output end of the clock inputbuffer 130 for generating the delayed clock output signal DCK. Besides,the delay lock loop circuit 140 also generates another delayed clockoutput signal DCKOUT, and the delayed clock output signal DCKOUT may bepresented by a differential signal pair.

In detail, the two gates logical circuit 110 may perform an OR operationon the specific mode signal SPCMD and the delayed clock output signalDCK to obtain a first result. The two gates logical circuit 110 alsoperforms a NAND operation on the first result and the clock enablesignal CLKENB to obtain the mode selecting signal MSS. Please noticehere, the two gates logical circuit 110 includes only two logic gates.

About the operation of the delay line ring oscillation apparatus 100,during the delay lock loop circuit 140 is in normal operation, the clockenable signal CLKENB is in logic low voltage level and the specific modesignal SPCMD is in high logic voltage level. Therefore, the modeselecting signal MSS is in logic high voltage level. Accordingly, thebuffer 120 receives the mode selecting signal MSS which is held on logichigh voltage level, and the buffer 120 generates the feedback signal FBwhich is also held on logic high voltage level. The clock input buffer130 receives the feedback signal FB which is also held on logic highvoltage level, and the input clock signal CKIN may be transported to thedelay lock loop circuit 140 by the clock input buffer 130.

In the embodiment of the disclosure, the clock input buffer 130 mayinvert the input clock signal CKIN to transport the inverted input clocksignal to the delay lock loop circuit 140.

On the other case, during the delay lock loop circuit 140 is on inprotection operation (for example, when the DRAM is in a power downmode, a self-refresh mode or in a NBTI (negative bias temperatureinstability) test mode), the specific mode signal is in logic lowvoltage level and the clock enable signal CLKENB is in high logic level.At this time, the gate numbers of the two gates logical circuit 110, thebuffer 120 and the clock input buffer 130 is odd, and a circuit loopformed by the two gates logical circuit 110, the buffer 120, the clockinput buffer 130 and the delay lock loop circuit 140 performs a ringoscillator, and the ring oscillator starts slow oscillation with astable duty cycle about 50%. That is, the duty cycle of the delayedclock output signal DCKOUT generated by the delay lock loop circuit 140is 50%.

It should be notice here, the frequency of the feedback signal FB may beadjusted by the buffer 120 according to the control signal CTRL. Thatis, the frequency of the delayed clock output signal DCKOUT may beadjusted correspondingly.

The delayed clock output signal DCKOUT may be transported to all therest delay lock loop circuit, the clock tree, the delay compensationcircuit and the delay lock feedback circuit model, and all the restdelay lock loop circuit, the clock tree, the delay compensation circuitand the delay lock circuit feedback model can receive the slowoscillation signal with 50% duty cycle. That is, the delay lock loopcircuit 140 is protected form asymmetrical NBTI degradation. Meanwhile,the delay line ring oscillation apparatus 100 consumes much less powerdue to the very slow oscillation frequency of the delayed clock outputsignal DCKOUT.

Please referring to FIG. 2, FIG. 2 is a block diagram of a delay linering oscillation apparatus 200 according to another embodiment of thepresent disclosure. The delay line ring oscillation apparatus 200includes a NOR gate NOR1, an inverter INV1, a two gates logical circuit210, a buffer 220, a clock input buffer 230, a coarse delay lock loop240 and a fine delay lock loop 250. In the embodiment, the two gateslogical circuit 210 is an and-or-inverter (AOI) gate AOI1, wherein, theAOI gate AOI1 has a NAND input end EI1, a first OR input end EI2 and asecond OR input end EI3, and an AOI output end EO1. The NAND input endEI1 is used to receive a clock enable signal CLKENB, the first OR inputend EI2 is used to receive a specific mode signal SPCMD, and the secondOR input end EI3 is used to receive a delayed clock output signal DCK ora fine delayed output signal FDCK. The AOI output end EO1 is used togenerate a mode selecting signal MSS.

On the other hand, the clock input buffer 230 is a NAND gate NA1. Theinput ends of the NAND gate NA1 receive the feedback signal FB and theinput clock signal CKIN. The output end of the NAND gate NA1 is coupledto the coarse delay lock loop circuit 240.

The coarse delay lock loop circuit 240 is also coupled to the fine delaylock loop circuit 250. The coarse delay lock loop circuit 240 generatescoarse delayed output signal pair CDCK1 and CDCK2, and the coarsedelayed output signal pair CDCK1 and CDCK2 are transported to an inputend pair of the fine delay lock loop circuit 250. The fine delay lockloop circuit 250 generates the fine delayed output signal FDCK accordingto the coarse delayed output signal pair CDCK1 and CDCK2. Wherein, thecoarse delayed output signal CDCK1 and CDCK2 are differential signals.

The NOR gate NOR1 has three input ends in this embodiment. The inputends of the NOR gate NOR1 respectively receive a power down signalPwrDN, a self-refresh signal Sref and a test mode for negative biastemperature instability (NBTI) signal tmNBTI. The NOR gate NOR1 is usedto generate the specific mode signal SPCMD.

When at least one of the power down signal PwrDN, the self-refreshsignal Sref and the test mode for NBTI signal tmNBTI is in logic highvoltage level, the specific mode signal SPCMD is in logic low voltagelevel, and when all of the power down signal PwrDN, the self-refreshsignal Sref and the test mode for NBTI signal tmNBTI is in logic lowvoltage level, the specific mode signal SPCMD is in logic high voltagelevel.

The inverter IV1 is coupled to the AOI gate AOI1. The inverter IV1receives a signal CLKEN and inverts the signal CLKEN to generate theclock enable signal CLKENB. The clock enable signal CLKENB is providedto the NAND input end EI1 to the AOI gate AOI1.

Please referring to FIG. 3, FIG. 3 is a circuit diagram of the buffer220 according to the embodiment of the present disclosure. The buffer220 includes a plurality of inverting gates 310-340. The inverting gates310-340 are coupled in series. The inverting gate 310 includestransistors M1-M4, switches SW1 and SW2 and capacitors C1 and C2. Theinverting gate 320 includes transistors M5-M8, switches SW3 and SW4 andcapacitors C3 and C4. The inverting gate 330 includes transistorsM9-M12, switches SW5 and SW6 and capacitors C5 and C6. The invertinggate 340 includes transistors M13-M16, switches SW5 and SW6 andcapacitors C5 and C6.

In inverting gate 310, a first end of the transistor M1 is coupled to afirst reference voltage VCC1, and a second end of the transistor M1 iscoupled to a first end if the transistor M3. Control ends of thetransistors M1 and M2 are coupled together to form an input end of theinverting gate 310. A second end of the transistor M2 is coupled to afirst end of the transistor M4 and forms an output end of the invertinggate 310. A second end of the transistor M4 is coupled to a first end ofthe transistor M2. A second end of the transistor M2 is coupled to asecond reference voltage GND. The first reference voltage VCC1 may be anoperation voltage of the buffer 220, and the second reference voltageGND may be a ground voltage of the buffer 220.

The switch SW1 is coupled between the input end of the inverting gate310 and a control end of the transistor M3. The switch SW1 is used toconnect the control end of the transistor M3 to the input end of theinverting gate 310 or to the second reference voltage GND according tothe control signal CTRL. The switch SW2 is coupled between the input endof the inverting gate 310 and a control end of the transistor M4. Theswitch SW2 is used to connect the control end of the transistor M4 tothe input end of the inverting gate 310 or to the first referencevoltage VCC1 according to the control signal CTRL. The input end of theinverting gate 310 is used to receive the mode selecting signal MSS.

The statuses of the switches SW1 and SW2 are complementary. That is,when the switch SW1 connects the control end of the transistor M3 to theinput end of the inverting gate 310, the switch SW2 connects the controlend of the transistor M4 to the first reference voltage VCC1. On thecontrary, when the switch SW1 connects the control end of the transistorM3 to the second reference voltage GND, the switch SW2 connects thecontrol end of the transistor M4 to the input end of the inverting gate310.

The capacitor C1 is coupled between the output end of the inverting gate310 and the first reference voltage VCC1. The capacitor C2 is alsocoupled between the output end of the inverting gate 310 and the firstreference voltage VCC1.

All of the circuits of the inverting gates 310-340 are the same, and thedetail circuit structure of each of the inverting gate 320-340 are notdescribed here. Beside, the output end of the inverting gate 340generates the feedback signal FB. The buffer 220 may generate thefeedback signal FB by adjusting the frequency of the mode selectingsignal MSS according to the control signal CTRL. The number of theinverting gates may be changed according to the frequency of the modeselecting signal MSS, four inverting gates 310-340 used in the buffer220 is only an example.

In the embodiment, the transistors M1, M3, M5, M7, M9, M11, M13 and M15are P-type transistors, and the M2, M4, M6, M8, M10, M12, M14 and M16are N-type transistors.

Please referring to FIG. 4A and FIG. 4B, FIG. 4A and FIG. 4Brespectively are circuit diagrams of the capacitors C1 and C2 accordingto the embodiment of the present disclosure. The capacitors C1 and C2may be formed by transistors. The capacitors C1 is formed by a N-typetransistor MC1, and a drain and a source of the N-type transistor MC1are coupled to the first reference voltage VCC1, and a gate of theN-type transistor MC1 is coupled to the output end of the inverting gate310. The capacitors C2 is formed by a P-type transistor MC2, and a drainand a source of the P-type transistor MC2 are coupled to the firstreference voltage VCC1, and a gate of the P-type transistor MC2 iscoupled to the output end of the inverting gate 310.

In summary, the present disclosure provides the delay line ringoscillation apparatus with the two gates logical circuit, a buffer and aclock input buffer to form a loop, and the loop is used to generate aslow oscillation signal with 50% duty ratio. Such as that, theasymmetrical degradation stress problem of the delay line ringoscillation apparatus can be solved, and the system performance can beextended by making the delay line ring oscillation apparatus safe fromnegative bias temperature instability (NBTI) and/or positive biastemperature instability (PBTI) hazard.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of theinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the invention covermodifications and variations of this invention provided they fall withinthe scope of the following claims and their equivalents.

What is claimed is:
 1. A delay line ring oscillation apparatus, comprising: a two gates logical circuit, having three input ends and an output end, two of the three input ends respectively receiving a clock enable signal and a specific mode signal, and the third input end further receiving a delayed clock output signal or a fine delayed output signal, the two gates logical circuit performing a logical operating on the clock enable signal, the specific mode signal and one of the delayed clock output signal and the fine delayed output signal for generating a mode selecting signal on the output end of the two gates logical circuit; a buffer, receiving the mode selecting signal and generating a feedback signal according to the mode selecting signal and a control signal; a clock input buffer, receiving the feedback signal and a input clock signal, the clock input buffer deciding whether to transport the input clock signal to an output end of the clock input buffer or not according to the feedback signal; and a delay lock loop circuit, receiving and delaying the signal on the output end of the clock input buffer for generating the delayed clock output signal, wherein, a frequency of the feedback signal is adjusted according to the control signal.
 2. The delay line ring oscillation apparatus according to claim 1, wherein the two gates logical circuit is an and-or-inverter (AOI) gate, the AOI gate has a NAND input end, a first OR input end and a second OR input end, and an AOI output end, the NAND input end receives the clock enable signal, the first OR input end receives the specific mode signal, and the second OR input end receives the delayed clock output signal or the fine delayed output signal.
 3. The delay line ring oscillation apparatus according to claim 1, wherein the clock input buffer is a NAND gate, the NAND gate has a first input end a second input end and an output end, the first input end of the NAND gate receives the feedback signal, the second input end of the NAND gate receives the input clock signal, and the output end of the NAND is coupled to the delay lock loop circuit.
 4. The delay line ring oscillation apparatus according to claim 1, wherein the delay lock loop circuit is a coarse delay lock loop circuit.
 5. The delay line ring oscillation apparatus according to claim 4, further comprising: a fine delay lock loop circuit, having an input end pair, the input end pair being coupled to the coarse delay lock loop circuit, the fine delay lock loop circuit receiving a coarse delayed output signal pair from the coarse delay lock loop circuit through the input end pair, and the fine delay lock loop circuit generating the fine delayed output signal according to the coarse delayed output signal pair.
 6. The delay line ring oscillation apparatus according to claim 1, further comprising: a NOR gate, coupled to the two gates logical circuit, wherein the NOR gate receives a power down signal, a self-refresh signal and a test mode for negative bias temperature instability (NBTI) signal, the NOR gate is used to generate the specific mode signal.
 7. The delay line ring oscillation apparatus according to claim 1, wherein the buffer comprises: a plurality of inverting gates, the inverting gates are coupled in series, the first inverting gate receives the mode selecting signal, and the last inverting gate generates the feedback signal, each of a plurality of gate delays of each of the inverting gates is controlled by the control signal.
 8. The delay line ring oscillation apparatus according to claim 7, wherein each of the inverting gates comprises: a first transistor, has a first end, a second end and a control end, the first end of the first transistor is coupled to a first reference voltage, the control end of the first end is an input end of the inverting gate; a second transistor, has a first end, a second end and a control end, the control end of the second transistor is coupled to the control end of the first transistor, the second end of the second transistor is coupled to a second reference voltage; a third transistor, has a first end, a second end and a control end, the first end of the third transistor is coupled to the second end of the first transistor, the second end of the third transistor is an output end of the inverting gate; a first switch, coupled between the input end of the inverting gate and the control end of the third transistor, the first switch is used to connect the control end of the third transistor to the input end of the inverting gate or the second reference voltage according to the control signal; a fourth transistor, has a first end, a second end and a control end, the first end of the fourth transistor is coupled to the output end of the inverting gate, the second end of the fourth transistor is coupled to the first end of the second transistor; and a second switch, coupled between the input end of the inverting gate and the control end of the fourth transistor, the second switch is used to connect the control end of the fourth transistor to the input end of the inverting gate or the first reference voltage according to the control signal.
 9. The delay line ring oscillation apparatus according to claim 8, wherein each of the inverting gates further comprises: a first capacitor, coupled between the output end of each of the inverting gates and the first reference voltage; and a second capacitor, coupled between the output end of each of the inverting gates and the first reference voltage.
 10. The delay line ring oscillation apparatus according to claim 9, wherein the first capacitor is formed by a N-type transistor, the second capacitor is formed by a P-type transistor.
 11. The delay line ring oscillation apparatus according to claim 8, wherein the first transistor and the third transistor are P-type transistors, the second transistor and the fourth transistor are N-type transistors.
 12. The delay line ring oscillation apparatus according to claim 8, wherein when the first switch connects the control end of the third transistor to the input end of the inverting gate, the second switch connects the control end of the fourth transistor to the first reference voltage, when the first switch connects the control end of the third transistor to the second reference voltage, the second switch connects the control end of the fourth transistor to the input end of the inverting gate.
 13. The delay line ring oscillation apparatus according to claim 8, wherein the amount of the inverting gates of the buffer is even. 